As optical information recording mediums to record/reproduce information by irradiation with laser light, a magneto-optical disk (MO), a write-once read-many compact disk (CD-R), a rewritable compact disk (CD-RW), a write-once read-many digital video disk (DVD-R), a rewritable digital versatile disk (DVD-RAM), a rewritable digital versatile disk (DVD-RW) and the like have been generally known. As means for raising recording densities in the optical information recording mediums, land/groove recording has been known in which the recording is performed in recording layer portions corresponding to both a flat portion (land) between adjacent guide grooves for tracking, the guide grooves being formed into substantially circular shapes in parallel with one another in a substrate surface, and the inside (groove) of each guide groove (JP(A)-57-50330, JP(A)-9-73665, JP(A)-9-198716, JP(A)-10-64120 and the like).
Moreover, in recent years, as a method of raising a recording density, a technique has been proposed in which a numerical aperture (NA) of an objective lens of an optical head constituting an apparatus for recording/reproducing information is raised to about 0.85. When the NA is raised, a beam diameter in condensing laser light can be reduced, and therefore it is possible to record/reproduce a finer mark. When the NA is raised in this manner, instead of applying the laser light to a recording layer through a substrate having a thickness of 0.6 to 1.2 mm as in a conventional technique, a light-transmitting layer having a thickness of about 0.1 mm is formed on the recording layer of the optical information recording medium, and the laser light can be applied to the recording layer on the substrate via the light-transmitting layer to record and reproduce information.
It is considered that the recording density is significantly increased by combining these techniques, that is, by performing the land/groove recording by use of a high-NA optical head.
However, according to findings of the present inventors, in the case where the land/groove recording is performed using the high-NA optical head, there is a problem that an optical resolution differs with the recording in a recording layer portion corresponding to the flat portion between the guide grooves and the recording in a recording layer portion corresponding to the inside of the guide groove. Concretely, when the recording is performed in the recording layer portion corresponding to the flat portion between the guide grooves, a drop of a signal amplitude (on the basis of the signal amplitude of a long mark) becomes more remarkable following a decrease of mark length, as compared with a case where the recording is performed in the recording layer portion corresponding to the inside of the guide groove.
FIG. 5 is a diagram showing a relation between the mark length shown on the abscissa and the signal amplitude shown on the ordinate. This figure shows a result of the recording performed with respect to an optical disk having a phase change type recording layer by the use of an optical head having a wavelength of 405 nm and NA=0.85. In the phase change type optical disk used in the present experiment, a phase difference between reflected lights before/after the recording is substantially 0. A line segment denoted with reference numeral 27 shows a case where the recording is performed with respect to a portion corresponding to the inside of a guide groove, and a line segment 28 shows a case where the recording is performed with respect to one end portion of a portion corresponding to a flat portion between the guide grooves. When the signal amplitude in a short mark remarkably drops in the recording into the portion corresponding to the flat portion between the guide grooves, a sufficient signal quality is not obtained, and therefore a problem has occurred that the high-density recording cannot be performed. An optical resolution in a case where the recording is performed with respect to the portion corresponding to the flat portion between the guide grooves needs to be improved in order that the recording is performed with respect to the portion corresponding to the inside of the guide groove and the portion corresponding to the flat portion between the guide grooves in such a manner as to raise the recording density.
It is to be noted that this problem is not limited to the only case where the information is recorded and reproduced with respect to the recording layer through the light-transmitting layer. Even in a case where the laser light is applied to the substrate from its back surface in the same manner as in the conventional DVD, when the recording density is raised, that is, when a shortest mark length recorded into the disk shortens, the problem becomes remarkable. The remarkable drop of the signal amplitude in the short mark does not depend on whether the recording is performed into the portion corresponding to the flat portion between the guide grooves or the portion corresponding to the inside of the guide groove, and depends on an incidence direction of the laser light. That is, when the recording layer is irradiated with the laser light through the light-transmitting layer, the drop of the signal amplitude in the short mark recorded in the portion corresponding to the flat portion between the guide grooves becomes remarkable. When the laser light is applied through the substrate, the signal amplitude drop of the short mark recorded in the portion corresponding to the inside of the guide groove becomes remarkable.